Pastes commonly used in making lead-acid pasted-plate batteries are prepared by mixing an active material comprising finely divided lead oxide or a blend of oxides which may contain metallic lead in powder form and/or other additives with an aqueous solution of sulfuric acid, i.e., a dilute solution of sulfuric acid. This produces reactions that result in the formation of lead sulfate and the liberation of heat. The quantity of heat generated, of course, depends upon the rate of addition of the acid and the specific gravity (Sp.Gr.) of the dilute solution of sulfuric acid. The lead sulfate expands the paste, i.e. produces "bulking", which has an important effect on the operating characteristics of the finished battery; too little expansion results in hard, dense plates which limit the amperehour capacity of the battery. Frequently this causes the battery to fail in service by buckling of the positive plates, or sulfation of the negative plates. Alternatively, too great expansion of the paste results in shedding of the positive active material thus shortening the useful life of the battery. To provide for consistency of the paste during manufacture, it is customary to control the wet density or "cube weight" of the paste used in the plates. The term "cube weight" refers to the grams per cubic inch, i.e. g/in.sup.3, of wet paste. While this varies with the processes used by different manufacturers in lead-acid batteries it normally is within the range of 60 to 75 grams per cubic inch for positive plate pastes and about 65 to 80 grams per cubic inch for negative plate pastes.
Experience has shown that the ampere-hour capacity of lead-acid batteries increases, i.e. the efficiency of the electrically formed active mass rises, with an increase in the amount of sulfuric acid and/or water used in the preparation of the paste. This increases the porosity and thus the surface area of the active mass. However, the more sulfuric acid that is added the more difficult it becomes to mix the paste because of localized over-heating and, in addition, the formed active mass becomes so expanded and loose as to result in shedding of the active material thereby shortening the useful life of the battery. Thus in conventional paste manufacturing processes either the capacity or useful life of the battery had to be reduced. Similar difficulties have been encountered in the preparation of the active mass in tubular battery plates.
To overcome the foregoing difficulties it has heretofore been proposed to add fibrous or other bonding materials to storage battery active materials. Numerous plastic type bonding materials such as polyethylene, polypropylene, polystyrene and polyvinylchloride have been disclosed as for example in U.S. Pat. Nos. 3,099,586 3,184,339 and 3,228,796. Preformed fibers of such materials as well as polyester, glass and carbon have also been added to battery paste mixes. In U.S. Pat. No. 3,466,193 there is disclosed the use of lead fibers and "Dynel" fibers, an acrylonitrile-vinyl chloride copolymer, in a lead-acid battery paste mixture.
Linear fluorocarbon polymers or polyfluoroethylene and in particular polytetrafluoroethylene have been added to fuel cell electrodes because of their non-wetting properties. According to one typical prior art electrode forming technique, a fluorocarbon polymer may be incorporated in a cell plate by mixing the polymer with a particulate active material to form an aqueous paste. Since fluorocarbons are hydrophobic, this is accomplished by first dispersing the fluorocarbon in an aqueous solution containing a minor amount of a surface active agent usually less than 5% by weight. The surfactant allows the polymer to be uniformly dispersed in the water, so that in the pasty mixture of water surfactant, active material particles and polymer, the latter is uniformly distributed. The aqueous paste is spread onto a current collector to form a cell plate and the cell plate is then heated to drive off the water. After drying is completed, the cell plate is then heated to a temperature at or near which the polymer sinters. This performs the dual functions of decomposing the surfactant to drive it from the cell plate and sintering the polymer to give it a permanent set. Examples of this technique are disclosed in U.S. Pat. Nos. 3,419,900 and 3,385,736.
In another technique, also commonly practiced in forming electrodes, the active material may be first associated with the current collector which acts to hold the electrode material into a coherent body and the body is then impregnated with an aqueous dispersion of the fluorocarbon polymer. Drying and/or sintering are accomplished in the same manner. An example of this technique is disclosed in U.S. Pat. No. 3,451,856.
In U.S. Pat. No. 3,630,781 there is disclosed a process of forming rechargeable electrodes utilizing unsintered fluorocarbon binder. Polytetrafluoroethylene (PTFE) in an aqueous dispersion ("Teflon"-30) is mixed with a finely divided electrochemically active rechargeable electrode material, such as zinc, zinc-oxide, cadmium, cadmium-oxide, nickel-oxide, copper, copper-oxide, silver, silver-oxide or mercuric-oxide to form a paste. The aqueous dispersion is then broken by various means including drying at a temperature which does not exceed the boiling point of the aqueous carrier, freezing, solvent extraction or by increasing the pH of the carrier, as by introducing an alkaline reagent. In irreversibly breaking the dispersion of the PTFE, the PTFE becomes coagulated into nonsintered fibrous strands which are left within the paste to act as a binder.
In U.S. Pat. No. 3,666,563 there is disclosed a process for fabricating a fuel cell electrode which includes producing heat, in situ, sufficient to soften "Teflon" or some other polymeric thermoplastic suitable for forming adhesive particle-to-particle bonds. The process may be used to bond together the catalyst-containing particles of fuel cell electrodes described in U.S. Pat. No. 3,429,750. In the process, phosphoric acid or any acid having an ionization constant equal to or less than the second ionization constant of phosphoric acid is added to a mixture of electrode components, including supported catalyst-containing particles, a material exothermally reactive with the acid, and a polymeric thermoplastic such as "Teflon". Initially, upon addition of the acid to the mixture, some water is formed and some heat is given off by the reaction between the most readily ionizable hydrogen ions of the acid and a minor portion of the exothermally reactive material, but, the heat is insufficient to cause the polymeric thermoplastic to soften. The acid mixture is then heated to a temperature sufficient to set off a reaction between the acid and the exothermally reactive material, causing the polymeric thermoplastic, as stated in the patent, to become fluid and adherent thus bonding the catalyst-containing particles together. The heated mixture is then subjected to pressure to further bond the components together to form a fuel cell electrode.
In U.S. Pat. No. 3,898,099 there are disclosed battery electrode structures comprising unsintered polytetrafluoroethylene and active material wherein the polytetrafluoroethylene constitutes from 0.1 to 3% of the combined weight of the polytetrafluoroethylene and the active material. The electrodes are formed from a blend including powdered active material and dry powdered polytetrafluoroethylene with about 100-900% by weight excess lubricant such as mineral spirits. In one example, lead oxide and from 0.1 to 1% polytetrafluoroethylene powder are mixed with excess mineral spirits and filtered. The filtered mix is then worked by rolling for 30 minutes. The lead oxide-polytetrafluoroethylene mix is then pressed on an expanded metal grid and thereafter pressed at two tons, repressed and air dried. The electrode was discharged against a lead anode in sulfuric acid.
While known prior art methods have reduced the density of battery active material which resulted in improved utilization of active material, they have left something to be desired because they always resulted in a decreased life. In the present invention the paste density is decreased, the active material utilization is increased and the life of the resulting battery plates is at least equivalent to or greater than plates made with standard density paste.